Exchange 2000::Orphaned MailboxPROBLEM:
An active directory user object was erroneously deleted. The MMC where this misdeed took place did not have the Exchange Admin plug-in installed. The mailbox still existed, but was pointed to a reference to a user object that no longer exists. Can't be "reconnected" through Exchange System Manager since the mailbox was never marked for deletion.
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A user deletion was attempted properly (from Exchange Admin MMC plugin with the proper attempt to remove mailbox) but it didn't delete the mailbox. So the mailbox is associated with a non-existent user.
Right-Click the "mailbox" container under the information store in Exchange System Manager. Choose "run cleanup agent." Now the mailbox will appear with a red X icon. The mailbox may now be reconnected with another user or purged manually.